Fluorotellurite glasses and their use in telecommunication system components.
Optical components, in particular components employing optical fibers doped with rare earth metal ions, are widely used in telecommunication systems. A major application is in signal amplifiers which employ fluorescent ion emission for amplification of a signal. The ion emission occurs within the same operating wavelength region as the signals. Pump energy excites the rare earth metal ion causing it to fluoresce and thereby provide optical gain.
Glasses, doped with a rare earth metal ion and pumped with appropriate energy, exhibit a characteristic, fluorescence intensity peak. The evolution of telecommunication systems, with their increasing demands for bandwidth, has created a need for a rare-earth-doped, amplifier material having the broadest possible emission spectrum in the wavelength region of interest. It is a purpose of the present invention to meet this need.
The bandwidth of a fluorescent intensity curve is, rather arbitrarily, taken as the full width half maximum (FWHM) of the curve in nanometers wavelength. This value is the lateral width of the curve at one half the maximum intensity, that is, at one half the vertical height of the peak of the curve. Unfortunately, many glasses, that exhibit a fluorescence in an appropriate region, exhibit a rather narrow bandwidth. It is a further purpose of the invention to provide a family of glasses that exhibit a relatively broad bandwidth.
It is well known that glasses doped with erbium can be caused to emit fluorescence in the 1520-1560 nm. region. This enables a signal operating in this wavelength range to be amplified. The significance of the 1550 nm. wavelength in optical communication has led to extensive studies regarding the behavior of erbium as a rare earth metal dopant in glasses. It has also led to the study of a variety of glasses as the host for the erbium ion.
Silica glasses, doped with erbium and co-doped with aluminum, have been used to produce optical fibers that are capable of providing gain at 1.5 xcexcm. Recently, glass fibers have been reported that provide greater bandwidth at 1.5 xcexcm, and a flatter gain spectrum, than the silica glass fibers. These fibers are produced from a fluorozirconate glass doped with erbium and known by the acronym ZBLAN.
The fluorozirconate glass is more difficult to fabricate into a low loss fiber, and has poorer chemical durability, than the silica glasses. Consequently, it would be desirable to provide a glass that combines the optical properties of the fluorozirconate glass with the physical properties of the silica glasses. It is a basic purpose of the present invention to provide such glasses.
The literature reports that glasses, composed of ZnF2, PbO and TeO2, exhibit excellent transparency in the 3 to 18 xcexcm region of the spectrum. This has led to proposals that these glasses be used in such products as IR domes, filters and laser windows. Accordingly, extensive studies have been made to determine electrical and physical properties of representative glasses.
It is also known that glasses with moderately low maximum phonon energies, when doped with thulium or holmium, can display fluorescence in the vicinity of 1450 nm and 1650 nm respectively. These wavelengths lie outside of the currently used telecommunications band. However, they lie within the transparency window of most commercial optical fiber. With the ever-increasing demand for useful bandwidth, there will be a need for additional amplifier devices that operate over the remaining portions of this window that are not covered by erbium.
As the concentration of a rare earth metal ion, such as erbium, is increased, the optical gain increases up to a certain point. Beyond this point, the fluorescent signal is quenched, and the optical gain decreases. This phenomenon is considered to result from the dopant, rare earth metal ions interacting with each other in a manner commonly referred to as clustering. It is another purpose of the invention to provide a family of glasses which is readily capable of dissolving erbium ions, and which exhibits a broad bandwidth, thus indicating that clustering is inhibited.
The invention resides, in part, in a family of fluorotellurite glasses that consist essentially of, as calculated in mole percent, 30-75% TeO2, 15-60% ZnF2 and 0.005-10% of an oxide of erbium, thulium or holmium.
The invention further resides in an optical component for a telecommunication system that is composed of a fluorotellurite glass that has a high thermal stability (Tx-Tg), that readily dissolves rare earth metal oxides, that has a wide FHWM, and that has a composition consisting essentially of, as calculated in mole percent, 30-75% TeO2, 15-60% ZnF2 and 0.005-10% of an oxide of erbium, thulium or holmium.